Miscibility and Cocrystallization in Homopolymer–Segmented Block Copolymer Blends

The amorphous phase behavior of blends of poly(butylene terephthalate) (PBT) and poly-(ester–ether) segmented block copolymers (PEE) was found to vary from completely immiscible to miscible, depending on the copolymer composition. The predictions of the Flory–Huggins relationship are in general agreement with the observed behavior when the interaction parameters are estimated from solubility parameters. The results of thermal analysis and small-angle X-ray scattering experiments strongly suggest that the PBT and PEE copolymers are capable of cocrystallization in the miscible blends under all crystallization conditions. The cocrystalline microstructure results from the complete miscibility and the blocky nature of the copolymer (i.e., the identical chemical and crystalline structures of the PEE hard segments and PBT). The crystallization rate of the copolymer in the miscible blends was found to be significantly enhanced due to the presence of PBT, and the resulting crystal thickness was found to be the same as that observed for PBT. Partially miscible blends of PBT with copolymers containing intermediate hard-segment concentrations formed distinguishable crystal populations, but the crystallization rate of the copolymer in these blends was also strongly influenced by the presence of PBT.